Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
Cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, cement compositions are deployed in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened, and hence substantially impermeable, cement therein that supports the pipe string in the center of the well bore and at the same time bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in such remedial cementing operations as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
Often the hydraulic cement must be placed within or next to a porous medium, for example earth strata surrounding the wellbore. In such a circumstance, water tends to filter out of the slurry and into the strata during placement prior to setting of the cement. A number of difficulties arise from an uncontrolled fluid loss of this type. Such difficulties include an uncontrolled setting rate, improper placement of the slurry, impaired strength properties, and contamination of the surrounding strata. Excessive fluid loss, inter alia, causes a cement composition to be prematurely dehydrated, which may limit the amount of cement composition that can be pumped and subsequently the time allowable for the pumping operation. This may also decrease the compressive strength of the cement composition and negatively impact bond strength between the desired cement composition and a subterranean zone, the walls of pipe string and/or the walls of the well bore. These conditions are all undesirable in oil and gas well cementing operations.
In order for such well cementing operations to be successful, the cement compositions utilized may include a fluid loss control component to reduce the loss of fluid, e.g., water, from the cement compositions when they contact permeable subterranean formations and zones. The effectiveness of a fluid loss additive is often related to the size or the molecular weight of the polymer. A “large” polymer, or a polymer with a higher molecular weight, generally is more effective in preventing excessive fluid loss from a cement slurry than a “small” polymer, or a polymer with a lower molecular weight. However, large polymers have a negative impact on the properties of the cement slurry. The most common problem associated with large polymers as additives is an unwanted and deleterious increase in viscosity of the cement slurry. Cement fluid loss additives are needed which prevent excessive fluid loss and which also impart little or no added viscosity to the cement slurry.
Current fluid loss additives when used in the form of a non-aqueous water-in-oil emulsion or dispersion, or as a solid additive include the tendency to viscosify a cement slurry upon addition at the wellsite resulting in a fluid viscosity that is outside the desired range and can have negative impacts on other components of the cement slurry such that those other components fail to operate at their intended level and/or in their intended capacity. Thus an ongoing need exists for improved fluid loss additives and methods of utilizing same.